Can Blender save Rendered image periodically
There is a trick that you can recombine images in post editing with blender.
Then you can combine 10 images or so, (using animated seed settings).

So you render a still image as animation images with lower sample rates and recombine them. it takes a bit of manual work to recombine, render speed is slightly lower (bhv rebuilds between frames). for the rest doable.
in the beginning some people hoped to reduce noise.

or in theory one could use this method to increasingly improve a real animation render if you store the images in different folders (by some scripting).

But you might also check the latest blender builds and the denoiser, unless you do a lot of diamont /glass reflections such a sample settings can be decreased a lot.

There is a addon/script that does this 
http://adaptivesamples.com/2013/07/22/progressive-animation-render-addon-and-image-stacking/

Most of all though i recomend to review your render settings, today a fairly reasonable render can be achieved with proper render settings:
https://www.youtube.com/watch?v=8gSyEpt4-60